1. Field of the Invention
The present invention relates to a magnetic recording apparatus provided with a microwave-assisted head for writing data signals to a magnetic recording medium having large coercive force to stabilize magnetization.
2. Description of the Related Art
Recently, as the recording density of a magnetic recording apparatus represented by a magnetic disk drive apparatus becomes higher, magnetic particles, constituting a magnetic recording layer of a magnetic recording medium such as a hard disk, have been made fine, improvement of materials and improvement of fine head processing have been achieved, and resultantly surface recording density has been remarkably improved. Further, magnetic recording apparatuses using a perpendicular recording technique are spreading, the recording technique attaining improvement of the surface recording density by magnetizing the recording layer to a direction perpendicular to the surface of a magnetic recording medium, and further improvement of the surface recording density in the future is also expected.
On the other hand, as a record bit or a magnetic particle has been made finer, thermal fluctuation tends to occur. To overcome this fluctuation, it is desirable to use magnetic particles with larger magnetic anisotropy energy and larger coercive force as material of the recording layer, because, when the thermal fluctuation occurs, a signal detected from a read head element of a thin-film magnetic head fluctuates, and resultantly an S/N (signal to noise ratio) is degraded and the signal may be vanished at the worst.
However, when using such magnetic particles with larger magnetic anisotropy energy as the material of the recording layer, the coercive force of the recording layer reaches a large value of 4 kOe or more. When performing saturation magnetic recording, recording magnetic field is generally required to be given by about two times the coercive force, and it has been therefore sometimes difficult to bring the recording layer into the saturation magnetization, that is, to record/erase magnetic data when a magnetic head with conventional performance is used.
For solving this problem, Xiaochun Zhu and Jian-Gang Zhu propose a technique in a document, “Bias-Field-Free Microwave Oscillator Driven by Perpendicularly Polarized Spin Current”, IEEE Transactions on Magnetics, Vol. 42, No. 10, pp. 2670-2672, October 2006, such that: an STO (spin torque oscillator) structured with a multilayered magnetic thin film is formed within a gap between a main pole and an auxiliary pole, and oscillation of the STO generates a microwave magnetic field in plane to induce a precession movement of magnetization, to thereby reduce the magnetic field for inversion in magnetization.
This document discloses apparently from dynamic magnetization analysis of the recording layer that the magnetic field for inversion in magnetization can be reduced by 40% with assistance of the microwave. However, the structure of the STO has the following problems:    (1) It is necessary to stack the magnetic thin film by 5 layers or more, which makes processes very complicated;    (2) A magnetic film with very high magnetic anisotropy is required for the STO to oscillate, but such a magnetic film has not yet been attained;    (3) According to this document, it is assumed that oscillation frequency is controlled only by current density fed into the STO and slight change in the current density causes an abrupt change of the oscillation frequency to thereby make the control difficult, and that temperature change also varies largely; and    (4) Control parameters of the microwave magnetic field to be generated are not obvious, and the amount of magnetic field is not also obvious. While the control parameter is assumed to be the current density for supplying energy, adjustment of the current density causes the oscillation frequency to be changed, which results in difficulty of arbitrarily controlling both parameters.
On the other hand, on purpose to more reduce the magnetic field for inversion in magnetization, Zihui Wang and Mingzhong Wu propose in a document, “Chirped-microwave assisted magnetization reversal”, Journal of Applied Physics 105, 093903, 2009, such that the microwave frequency is shifted during the inversion in magnetization, that is, a kind of frequency modulation is performed.
According to this document, the following problems occur.    (A) Though the magnetic field for inversion in magnetization is more remarkably reduced compared to the case that the frequency is kept constant, magnetic inversion time is about 2 ns in both cases, which is a large absolute value, and therefore data writing at high speed is difficult (high-speed inversion about 100 ps is required).    (B) When the frequency is shifted, convergence time of the magnetic inversion becomes slightly larger compared to the case that the frequency is kept constant.